Significant reduction in video bandwidth for storage and transmission purposes is desirable in various applications such as compact disc video and high-definition television. One type of video compression system which has received considerable attention lately is that proposed by the Moving Pictures Expert Group (MPEG), a committee within the International Standards Organization (ISO). The MPEG system is described in a paper entitled, "MPEG Video Simulation Model 3 (SM3)" by the Simulation Model Editorial Group, available from ISO as ISO-IEC/JTC1/SC2/WG11/N0010 MPEG 90/041, 1990 which is hereby incorporated by reference for its teachings on the MPEG video signal encoding method. This system is related to the Conditional Motion Compensated Interpolation (CMCI) video encoding system described in U.S. Pat. No. 4,999,705 entitled THREE DIMENSIONAL MOTION COMPENSATED VIDEO CODING, which is hereby incorporated by reference for its teachings on video encoding techniques.
The MPEG system integrates a number of well-known data compression techniques into a single system. These include motion-compensated predictive coding, discrete cosine transformation (DCT), adaptive quantization and variable-length coding (VLC). In these systems, the adaptive quantization step is performed on the coefficient values produced by the discrete cosine transform operation for blocks of 64 pixels derived from the input image.
The DCT coefficients are quantized with varying resolution as a function of the amount of data generated by the encoding operation. In a system with a fixed-bandwidth channel, if an individual image frame produces a relatively large amount of encoded data, the quantization step sizes applied to successive frames may need to be increased (made coarse) to reduce the amount of encoded data used to represent those frames. This is done so that the average level of data produced over several frame intervals is able to be transmitted through the fixed-bandwidth channel. If, when the quantizer is applying coarse quantization to the DCT coefficients, an image is encoded which includes an object having relatively few contours, the reproduced image of this object may have undesirable quantization distortion. This distortion would appear as an exaggeration of the contours in the object.
In addition, a larger quantization step size is usually applied to the DCT coefficients that represent the high diagonal spatial frequencies within the block that the DCT coefficients describe, since the human eye is less sensitive to detail along image diagonals than to horizontal or vertical detail. However, this technique may also produce contours in the overall image formed by the blocks since the quantization levels may vary from block to block depending on the varying amount of encoded information produced by surrounding blocks.